This invention relates to a navigation system for a mobile unit such as a car navigation system, and more particularly to a GPS receiver and a GPS position measurement method wherein GPS, (Global Positioning System) can be performed in a short time.
The GPS system is a position measurement system developed to allow a mobile unit such as an aircraft or a ship to determine the position on the earth or the velocity of the mobile unit on the real time basis utilizing GPS satellites which fly up in the sky. Recently, the GPS system is utilized widely in the field of the static survey for measuring the distance or the direction between different spots on the earth and the like fields in addition to the position measurement by a mobile unit. In order to utilize the GPS system, a GPS receiver for receiving radio waves radiated from GPS satellites is used.
FIG. 9A shows a general construction of a GPS system used popularly, and FIG. 9B illustrates a conventional GPS position measurement operation. Referring first to FIG. 9A, a spread spectrum signal of 1.57542 GHz is transmitted from a GPS satellite 200. An antenna section 211 of a GPS receiver 210 receives the transmitted signal after a propagation time which relies upon the distance between the GPS satellite 200 and the GPS receiver 210. The signal received by the antenna section 211 is down converted into a signal of a predetermined intermediate frequency by a radio frequency (RF) section 212 and then supplied to a signal synchronizing demodulation section 213. The intermediate frequency signal is despread into demodulated data by the signal synchronizing demodulation section 213. The demodulated data is used for position measurement calculation by a signal processing section 214. In this manner, the signal transmitted from the GPS satellite 200 is received and used for position measurement calculation by the GPS receiver 210.
FIG. 9B illustrates a conventional GPS position measurement operation performed by the GPS receiver 210. First, when power supply to the GPS receiver 210 is made available, a frequency search is performed. The frequency search is performed in order to synchronize a frequency of a low accuracy produced by a frequency oscillator 215 in the GPS receiver 210 and having an error with a signal frequency of a high accuracy transmitted from the GPS satellite 200. If some correlation between the frequencies is detected, then the GPS receiver 210 perform a PLL (Phase Locked Loop) operation for adjustment in phase to synchronize the internal frequency fully with that of the signal from the GPS satellite 200. Then, after an edge at a bit boundary is detected and therefore data can be fetched, time information is confirmed. In particular, the TOW (Time Of Week) placed in the second word of a subframe in a hierarchical navigation message and representative of a signal time within one week in the period of 6 seconds is fetched. After the TOW is fetched, position measurement calculation is started. After the position measurement calculation is completed, position measurement data is outputted, and the current position is calculated finally.
In this manner, the GPS position measurement method requires the frequency oscillator 215 for capturing a signal from the GPS satellite 200, and in order to establish synchronism with a signal frequency of a high accuracy transmitted from the GPS satellite 200, it is required that the frequency oscillator 215 is high in accuracy. However, the oscillation frequency of the frequency oscillator 215 is fluctuated generally by a temperature or a secular change of a quartz oscillator. This fluctuation prevents easy capture of the signal from the GPS satellite 200 through the use of the frequency oscillator 215, and therefore, a scheme of a frequency search must be provided separately. Since usually the frequency search requires much time, a considerably long time is required until the current position is calculated finally.
Further, in the conventional GPS position measurement method, the time required after the signal from the GPS satellite 200 is captured until all of absolute time information included in the signal is acquired is approximately 6 seconds even upon hot starting, with which the time is minimized, and in the best conditions, but usually, a time of tens and several seconds is required. Further, since position measurement calculation is performed using the acquired absolute time information, a considerably long time is required until the current position is calculated.
Furthermore, in the conventional GPS position measurement method, when position measurement is performed again after a time longer than a fixed interval of time elapses, time for fetching a navigation message newly is required. Therefore, a considerably long time is required until the current position is calculated.
Where much time is required for GPS position measurement from such reasons as described above, for example, in a car navigation system, the current position cannot be discriminated immediately after power supply is made available. This raises a problem that the route to a destination cannot be discriminated rapidly or the current position is unsettled due to an error of the self-contained navigation and this increases time until the correct position is discriminated. Further, in an apparatus of the type wherein a GPS receiver is built in or connected to a recent portable information terminal, if the apparatus is used principally during walking of the user, since the current position cannot be measured rapidly, the user must wait at a place with the apparatus held in hand until the position measurement is completed, which is very inconvenient.
On the other hand, it is also possible to perform position measurement with the power supply normally kept on. However, this causes the apparatus to consume very much power. Where the apparatus is particularly limited in power consumption like, for example, a car navigation system or a portable navigation system, it is not preferable to normally keep the power supply on.
Also where predetermined standby/startup of a GPS receiver is repeated to intermittently receive GPS signals, in order to maintain the accuracy in frequency and time, it is necessary to reduce the interval of intermittent receptions to perform position measurement frequently, resulting in increase of the power consumption.
It is an object of the present invention to provide a GPS receiver and a GPS position measurement method wherein GPS position measurement can be performed stably and rapidly without the necessity to wait for periodical time information from a GPS satellite. It is another object of the present invention to provide a GPS receiver and a GPS position measurement method wherein power consumption is minimized also with a minimized position measurement time through the selection of an optimum time interval between intermittent receptions of GPS signals.
In order to attain the objects described above, according to an aspect of the present invention, there is provided a GPS receiver, including a GPS block for receiving a signal transmitted from a GPS satellite and performing position measurement based on the received signal to update a navigation message and for repeating a standby state for reception and a startup state thereof, and an external clock block for storing frequency information and time information of a high accuracy, the external clock block outputting a standby signal and a start signal, which are to be transmitted to the GPS block, at predetermined time intervals based on a setting signal transmitted from the GPS block and the frequency information and the time information stored in the external clock block.
The GPS receiver may further include a frequency measurement block for measuring the difference of the frequency of a frequency oscillator of the GPS block from the frequency information held in the external clock block as a frequency offset, the frequency offset being outputted from the frequency measurement block to the external clock block. Further, the external clock block may output the standby signal and the start signal, which are to be transmitted to the GPS block, at predetermined time intervals based on the accuracy of the frequency information and the time information stored in the external clock block. With the GPS receiver, further reduction in power consumption can be anticipated.
The GPS receiver may further includes a frequency measurement block for measuring a frequency offset which is a displacement of a frequency oscillator of the GPS block with reference to the frequency information held in the external clock block and outputting the measured frequency offset to the external clock block. The external clock block may control standby and startup of the GPS block based on the frequency offset outputted from the frequency measurement block. With the GPS receiver, an optimum interval of intermittent GPS receptions can be obtained based on the frequency offset of the frequency oscillator, which allows augmentation in performance and further reduction in power consumption.
According to another aspect of the present invention, there is provided a GPS receiver, including a GPS block for receiving a signal transmitted from a GPS satellite and performing position measurement based on the received signal to update a navigation message and for repeating a standby state for reception and a startup state thereof, and a radio wave clock block for storing time information of a high frequency and frequency information of a high accuracy obtained through reception of a signal transmitted from a radio wave block broadcasting station, the radio wave clock block outputting a standby signal and a start signal, which are to be transmitted to the GPS block, at predetermined time intervals based on a setting signal transmitted from the GPS block and the time information and the frequency information stored in the radio wave clock block.
The radio wave clock block may output the standby signal and the start signal, which are to be transmitted to the GPS block, at predetermined time intervals based on the accuracy of the time information and the frequency information stored in the radio wave clock block. Where the radio wave clock block is used as the external clock block in this manner, the GPS receiver is advantageous in that, when the accuracy of the time information and the frequency information held in the radio wave clock block is high, the standby state of the GPS receiver can be extended in the maximum to a valid time of a navigation message and can be further extended to further reduce the power consumption.
According to a further aspect of the present invention, there is provided a GPS position measurement method for a GPS receiver, including the steps of receiving a hierarchical navigation message from each of a plurality of GPS satellites, storing the received navigation messages into a memory built in the GPS receiver, storing frequency information and time information of a high accuracy into an external clock which normally is in an operating state, and repeating a startup state and a standby state of the GPS receiver within a predetermined time in response to a standby signal and a startup signal outputted from the external clock based on the setting signal transmitted from the GPS receiver to the external clock and the frequency information and the time information stored in the external clock to perform position measurement of the GPS receiver from the GPS satellites to update the navigation messages stored in the memory.
Also with the GPS position measurement method for a GPS receiver, the GPS measurement time can be reduced significantly, and the necessity to wait for periodical time information from a GPS satellite is eliminated. Consequently, the time required before position measurement is stabilized and reduced. Further, since an optimum GPS reception interval can be selected in accordance with a situation of the external clock, reduction of power consumption can be anticipated.
Preferably, an interval of time between startup and standby of the GPS receiver is varied based on a degree of accuracy of the frequency information or the time information held by the external clock. With the GPS position measurement method, frequency information and time information of a high accuracy and a navigation message can be held with certainty without depending upon a situation of the external clock.
It is to be noted that, for example, if the external clock is constructed for reception of a carrier from a radio wave clock broadcasting station, the accuracy of the frequency information or the time information held by the external block allows elongation of the intermittent reception interval to the maximum even if the GPS receiver does not include a real time clock (RTC) of a high accuracy.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.